Limestone is a very common sedimentary rock of biochemical origin.
It is composed mostly of the mineral calcite. Sometimes it is almost pure
calcite, but most limestones are filled with lots of other minerals and sand
and they are called dirty limestones. The calcite is derived mostly from
the remains of organisms such as clams, brachiopods, bryozoa, crinoids and
corals. These animals live on the bottom of the sea and when they die their
shells accumulate into piles of shelly debris. This debris can then form
beds of limestone. Some limestones may have been derived from non-biogenic
calcite formation. Although some limestones can be nearly pure calcite, there
is often a large amount or sand or silt that is included in the shelly debris.

Limestones form usually close to the source of shelly debris although
some significant transport can occur. Great sources for limestone are reefs.
Reefs have been in existence for most of the history of life on Earth, but
they have changed in the species that build them. Stromatolites, which are
complex living structures of more than one organism (cyanobacteria and algae),
formed the first reef like structures in the Cambrian Period. Early reefs
in the Ordovician were composed of small crinoidal, bryozoan and brachiopod
reef communities. In the Devonian, reefs became extremely large with tabulate
and solitary corals starting to dominate, but brachiopods and crinoids still
significant contributors. Some Cretaceous reefs really took on some huge
proportions and were dominated by large, now extinct mollusks called rudist.
Since those times, modern corals and bivalves (clams) have been the prime
reef-building organisms.

All these carbonate shelled organisms needed the same requirements
out of their ocean environments: sunlight, a food source, and enough turbulence
to remove sand and clay. Where you find these conditions is usually the same,
on the margins of flat littoral surfaces. Reefs tend to be offshore from
sandy beaches but not in too deep of water to not have sunlight. In fact
reefs often build upon the skeletal debris of former reef inhabitants to
continually grow upward to the sea surface where turbulence keeps the reef
"clean" from sand and clay debris. Ancient reefs and limestone’s are closely
interconnected, although not all limestones indicate an ancient reef.

Because of limestone's biogenic origins, it is often the best rock
for finding fossils. The organisms themselves leave fossils in the rock and
entire communities and even entire reef structures can be preserved in a
limestone bed. At times a limestone is entirely composed of fossils. The
rock coquina is a variety of limestone and is composed entirely of fragments
of sea shells.

But most limestones have a significant amount of carbonate mud.
This mud matrix can even constitute 100% of the limestone rock. Origins of
this mud are debated and may just be a fine grained mud left from the erosion
and abrasion of calcite shells. There maybe a non-biogenic origin too. At
times modern carbonate muds can accumulate in the oceans in thick layers
that are destined for limestone formation. A limestone variety is caused by
swift currents that rolled carbonate mud into small beads that (once solidified)
look like tiny eggs. This limestone variety is called an oolite and is sometimes
very ornamental.

Limestone is almost always marine (ocean water) in origin and is
usually associated with other near shore rocks types. In a typical marine
scenario, to the shore side of a reef is the silica mud of a lagoon and closer
to shore is the sand of a beach. The silica mud will form a shale while the
beach sand will form a sandstone. Farther inland might be a swamp whose organic
debris might form a coal layer. Throughout the Carboniferous time period,
coals were often interbedded with sandstones, shales, and limestones in repeating
cycles. These cycles represent changes in ocean levels over thousands of
years as swamps are flooded by a beach and then a lagoon and perhaps a reef.
But as sea levels fall the limestone of the reef is replaced by the shales
of a lagoon and then the sandstone of a beach and on and on. Hundreds of
feet of repeating cycles like this can occur.

Limestones are important rocks. They can be used for building materials
but are not quite as strong as sandstone and are easily weathered by acidic
conditions. Limestones are the primary source of lime for cements. Cement
is considered one of the most important construction materials ever invented
by man. Limestone can be crushed and used as road ballast. Significant quantities
of limestone are quarried around the world for these purposes.

Limestone is usually the type of rock that gives us caves. Most
caves are the result of dissolution of calcite by acidic waters. Ground water
can dissolve portions of massive limestone formations and yield extremely
large caverns. Large caves and numerous sinkholes are often found in areas
that have significant limestone formations.

Metamorphosed, fairly pure limestone forms the metamorphic rock,
marble.
During the metamorphic process, the crystals of fine grained calcite in the
limestone become merged and melded into other large crystals forming the
interlocking course grained texture of the marble. All limestones under
go some kind of alteration after initial solidification. These alterations
can include dolomitization, recrystalization, styolitization, compaction,
cementation and exsolution to name a few. All of these things are considered
part of the diagenetic process. Diagenesis is anything that happens to a
sedimentary rock after original deposition. At some point diagenesis and
metamorphism meet and the stone is no longer a limestone, but a marble.
The boundary between the two is well studied and usually not difficult to
distinguish as significant changes occur to the calcite crystals during metamorphism.

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